mdadm: correctly set up testing location for ptests
1. Do not clutter /, create a special-purpose dir
2. Clean up the dir after tests are done (if this is not
performed, disk will overflow later in ptesting).
3. Fix up more locations in ptests to use the dir.
Upstream default /var/tmp is not suitable as it is not
big enough (mdadm needs about 500 Mb)
Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
